Color ink jet printers generally use cyan, yellow, and magenta inks to generate a full range of colors in a printed image. Many such printers use a separately replaceable black ink cartridge to enhance the accuracy of color image tones, and to generate portions of the output containing text and other black images. Because the black cartridge contains only one ink color, it may have an extended linear array of nozzles; given a limited number of data input lines to the cartridge, it may print a relatively wide swath in a single pass across a page. In contrast, the color cartridge must have independent inputs for each of the three colors. For the same given limited number of data inputs, a much narrower swath of color ink may be printed in a single pass. Therefore, the printing throughput rate is much slower for color printing than for black-only printing.
Ink jet printers achieve improved print uniformity by printing overlapping partial density swaths in a process known as "shingling." During the typical two-pass shingled printing, each swath prints only every other dot row of the final image, and every portion of the resulting fully printed output will have been printed with ink contributed by two passes of the print head. By this approach, a lower half of each new swath interlaces and fills in the remaining incomplete upper half of the previous swath, and provides a new extending partial density portion to be completed by pan of the next swath. Three- or four-pass shingling follows the same principle, providing even better print quality with slower printing speed.
If an area is printed by more passes than required, the excess ink will create a non uniformity. Therefore, efforts in existing printers simply to change feed rates in transition between color and black-only modes would result in over inking when switching to a slower feed rate for color printing, and under inking when changing to a faster feed rate.
Some existing printers provide full speed black-only printing for pages containing no color data, and revert to the slower color rate only on pages containing at least some color data. Other printers may use the faster black-only rate on a strictly limited class of pages including color images. Such a page must include a region that contains no color data, that extends the full page width, and that is separated from color-containing regions above and below by an unprinted horizontal white band of a nominal width. These requirements are so limited that there are common circumstances in which a largely black-only page containing only a minimal amount of color data must be printed entirely at the slower color throughput rate. For instance, a spread sheet having only one data cell highlighted in color, and the remaining text and grid printed in black without horizontal white bands, would be printed entirely at the slower rate.
The method disclosed herein overcomes these disadvantages by providing a method of generating an image on a printable surface from a stream of incoming print data containing data of a first data type for generating an image of a first image type and containing data of a second data type for generating an image of a second image type, the method comprising the following steps. While printing in a first mode, depositing a first partial density image swath having a first swath width on the printable surface to overlap at least a portion of a partial density image swath previously printed on the surface, thereby generating a shingle tab extending from the previous swath having a first length. After depositing the first swath, the surface is advanced by a first amount. While printing in the first mode, incoming print data is scanned to determine whether the incoming print data includes the second data type. If so, there is a transition to a second print mode, which includes depositing a second partial density image swath having a second swath width different from the first swath width on the printable surface to overlap at least a portion of a partial density image swath previously printed on the surface, and generating a shingle tab extending from the previous swath having a second length different from the first length. After depositing the second swath, the surface is advanced by a second amount different from the first advance amount.